First off, the sound difference between the 6505+ 112 and the 6505+ head through the same cabinet would be absolutely unidentifiable. Even if you can hear a difference, a free VH4 says that you couldn't accurately and consistently name which was which over a viariety of compared tones. The headroom difference, even coupled with a hotter bias, will not be THAT noticable. I've adjusted the bias from hot to cold on my amp, and as long as you're within the normal operation range of the tubes, you can't hear a difference that's any more significant that taking two steps to the left when you play. In addition, my amp has selectable power, and, once again, the tone difference is more likely in your head. The only major difference you can notice is that 60w is more managable at lower volumes, but BARELY.

As for the 5150 III, it's not even close to the same as a 5150, 5150 II, 6505, or 6505+. The only similarity is in the name and the relationship to Eddie. The 5153 is made under the brand EVH, which is made by Fender. The 5153 isn't anything close to the 5150/6505 or 5150 II/6505+. It's about as close to those two as a JCM2000 DSL100 is to a Plexi, or a Mesa Dual Rectifier is to a Mark V.

The Ironheart might be worth checking out. I know one of the guys on here just bought one. He complained about the boost being useless, though, and the effects loop isn't footswitchable, if that matters. I would take a 5153 over an Ironheart any day, though.